MOSFET amplifier having feedback controlled transconductance

ABSTRACT

A signal is applied to the body of a MOSFET to enhance the transconductance of the MOSFET. The signal applied to the body of the MOSFET has essentially the same waveform as an input signal supplied to the gate of the MOSFET, and is shifted by approximately 180 degrees with respect to the input signal. The signal applied to the body of the MOSFET may be provided by a phase-adjusting feedback circuit that generates the signal from a signal representing the output of the MOSFET.

RELATED APPLICATIONS

Priority is claimed from U.S. Provisional Application 60/550,527 filed 5Mar. 2004, the entirety of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to the field of electronic amplifiercircuits.

2. Description of Related Art

The growth of technologies such as wireless communication has led to anincreasing need for high performance electronic amplifiers that havehigh gain and low noise.

FIG. 1 shows a typical example of a low noise RF amplifier known as acascode amplifier. The cascode amplifier includes a first MOSFETtransistor 10 and a second MOSFET transistor 12 that are connected inseries between a load impedance Zload driven by a voltage source Vdd anda common potential (referred to herein as ground). The first transistor10 receives an input signal RFin at its gate along with a DC biasvoltage supplied by a bias circuit 14. The input signal RFin isamplified in the first transistor 10 to produced an amplified signal atthe drain of the first transistor. The amplified signal is received atthe source of the second transistor 12 and is conducted through thesecond transistor 12 to an output node where an output signal RFout ispresented. The second transistor 12 typically does not amplify thesignal provided by the first transistor 10, but rather is used toprevent the output node from seeing parasitic capacitances of the firsttransistor, thus lowering the output impedance of the amplifier circuitand improving its frequency response.

As seen in FIG. 1, the substrates of the MOSFET transistors 10, 12 arecoupled to their respective sources. This arrangement is utilized toeliminate the substrate bias effect or “body effect,” in which theMOSFET substrate (also referred to herein as the “body”) acts as asecond gate that influences carrier availability in the MOSFET channelregion. The body effect is explained in more detail with reference toFIGS. 2 and 3.

FIG. 2 shows a cross-section of a typical n-type MOSFET comprised of ap-type substrate 20 in which n-type source and drain regions 22, 23 areformed at opposing sides of a channel region 24. For purposes of thepresent explanation, the channel region 24 is shown as having an n-typeinversion layer formed therein. A gate dielectric 26 lies between thechannel region 24 and a gate electrode 28. A depletion layer 29separates the p-type and n-type regions. The thickness of the depletionlayer with respect to the other elements is exaggerated for purposes ofillustration.

The drain current of the MOSFET is controlled by modulating theavailability of majority carriers (in this case, conduction bandelectrons) in the channel region between the source and drain. Carrieravailability is largely controlled through a capacitive effect that iscaused by application of a voltage to the gate 28. Consequently,variations in the gate voltage produce corresponding variations incarrier availability that cause the drain current to be modulated in amanner that corresponds to modulation of the gate voltage. However,carrier availability is also affected in a similar manner by any voltageapplied to the body of the MOSFET. Thus, the MOSFET is typically modeledin the manner shown in FIG. 3. In this model, the behavior of the MOSFETis approximated by a pair of parallel connected current sources 30, 32that produce a drain current Id. The first current source 30 representsthe effect of the gate voltage Vg on carrier availability, whichproduces a current having a magnitude approximately equal to the MOSFETtransconductance Gm times the gate-source voltage Vgs. The secondcurrent source 32 represents the effect of the body voltage Vb oncarrier availability, which yields a current having a magnitudeapproximately equal to the body effect transconductance Gmb times thesource-body voltage Vsb. As shown by this model, the application of areverse bias to the source-body junction (i.e., a voltage that widensthe depletion layer 29) has an effect that is equivalent to thegeneration of a current in the channel region that is opposite inpolarity to the current produced in response to the gate voltage,resulting in an over-all reduction in the drain current produced inresponse to a given gate voltage. Since the MOSFET body is typicallyheld at a fixed voltage, the body effect is generally understood toreduce the transconductance of the MOSFET or to increase the thresholdvoltage of the MOSFET. In order to avoid this effect, MOSFET circuitssuch as the cascode circuit of FIG. 1 connect the source directly to thebody so that the source-body voltage is zero.

SUMMARY

The transconductance of a MOSFET may be enhanced by applying a signal tothe body of the MOSFET. The signal has essentially the same waveform asan input signal supplied to the gate of the MOSFET, and is shifted byapproximately 180 degrees with respect to the input signal. In theexemplary embodiments described herein, the signal is a feedback signalthat is derived from the output of the MOSFET and that is phase-invertedwith respect to the signal applied to the gate of the MOSFET.Application of the phase-inverted signal to the body of the MOSFETcontrols carrier availability in a manner that enhances the effectiveMOSFET transconductance.

In accordance with one embodiment, an amplifier circuit comprises aMOSFET that receives a periodic input signal at its gate and amplifiesthe input signal to produce an output signal. A phase-adjusting feedbackcircuit receiving a signal corresponding to the output signal andapplies a phase-adjusted signal to the body of the MOSFET. Thephase-adjusting feedback circuit causing a phase shift of the receivedsignal such that the phase-adjusted signal applied to the body of theMOSFET is shifted by approximately 180 degrees with respect to theperiodic input signal of the MOSFET.

In accordance with another embodiment, a cascode amplifier circuitcomprises a first MOSFET, a second MOSFET having its source connected tothe drain of the first MOSFET, and a feedback circuit coupled betweenthe gate of the second MOSFET and the body of the first MOSFET.

In accordance with another embodiment, a differential cascode amplifiercircuit comprises a first MOSFET, a second MOSFET having its sourceconnected to the drain of the first MOSFET, a third MOSFET, and a fourthMOSFET having its source connected to the drain of the third MOSFET. Afirst feedback circuit is coupled between the gate of the second MOSFETand the body of the first MOSFET, and a second feedback circuit coupledbetween the gate of the fourth MOSFET and the body of the third MOSFET.

In accordance with another embodiment, a differential cascode amplifiercircuit comprises a first MOSFET, a second MOSFET having its sourceconnected to the drain of the first MOSFET, a third MOSFET, and a fourthMOSFET having its source connected to the drain of the third MOSFET. Afirst feedback circuit is coupled between the gate of the second MOSFETand the body of the third MOSFET, and a second feedback circuit iscoupled between the gate of the fourth MOSFET and the body of the firstMOSFET.

In accordance with another embodiment, a method of producing anamplified signal comprises providing a first signal as an input signalat the gate of a MOSFET, applying a second signal to the body of theMOSFET, where the second signal has approximately the same waveform asthe first signal and is shifted by approximately 180 degrees withrespect to the first signal, and providing an output signal at a drainof the MOSFET in response to the first signal and the second signal.

Other features and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings which illustrate, by way of example, variousfeatures of embodiments of the invention.

DESCRIPTION OF THE DRAWINGS

A detailed description of embodiments of the invention will be made withreference to the accompanying drawings, wherein like numerals designatecorresponding parts in the several figures.

FIG. 1 shows a schematic diagram of a prior art cascode amplifiercircuit.

FIG. 2 shows a cross-section of a typical n-type MOSFET.

FIG. 3 shows a model of a MOSFET that represents the relationship of thebody effect to effective MOSFET transconductance.

FIG. 4 shows a generalized diagram of a MOSFET amplifier circuit inwhich feedback is used to enhance transconductance.

FIG. 5 shows waveforms illustrating an effect that may be achieved byapplying a phase-adjusted feedback signal to the MOSFET body.

FIG. 6 shows a schematic diagram of a MOSFET amplifier circuit havingenhanced transconductance according to a first embodiment of the presentinvention.

FIG. 7 shows a schematic diagram of a MOSFET amplifier circuit havingenhanced transconductance according to a second embodiment of theinvention having a differential topology.

FIG. 8 shows a schematic diagram of a MOSFET amplifier circuit havingenhanced transconductance according to a third embodiment of theinvention having a differential topology.

DETAILED DESCRIPTION

In the following description reference is made to the accompanyingdrawings in which are shown specific embodiments through which theinvention may be practiced. It is to be understood that otherembodiments may be implemented and changes may be made without departingfrom the scope of the claimed invention.

FIG. 4 shows a generalized diagram of a MOSFET amplifier circuit inwhich feedback is used to enhance the effective MOSFET transconductance.The circuit is comprised of an n-type MOSFET 40 arranged as a commonsource amplifier. The gate of the MOSFET 40 receives an input signalRFin and a bias signal supplied by a bias circuit 42. The drain of theMOSFET receives a bias from a voltage source Vdd through a loadimpedance Zload. An output signal RFout is produced at the drain. Aphase-adjusting feedback circuit 44 receives an input signal that isderived from the drain current of the MOSFET 40 and produces an outputsignal that is phase-adjusted so as to be shifted by approximately 180degrees with respect to the MOSFET input signal RFin. The phase-adjustedoutput signal is applied to the body of the MOSFET. This results inenhancement of the effective transconductance of the MOSFET.

The effect of the phase-adjusted feedback signal on transconductance isnow explained in more detail. First, as shown in the model of FIG. 3,the MOSFET transconductance corresponding to the gate voltage isrepresented by Gm, and the body effect transconductance is representedby Gmb. It is inherent that the body effect transconductance Gmb isrelated to the MOSFET transconductance Gm, and therefore the followingrelationship is defined:Gmb=αGm (α is typically 0.3)   (1)

In the model of FIG. 3, the drain current Id of the MOSFET amplifier isrepresented as:Id=GmVgs−GmbVsbHowever, the model of FIG. 3 assumes that Vsb is a static value thatranges from zero to an arbitrary value that applies a reverse bias tothe source-body junction and that therefore has the static effect ofreducing carrier availability under all conditions. In contrast, in thecircuit of FIG. 4, the value Vsb is a dynamic value that alternatelyprovides a reverse bias or a forward bias with respect to thesource-body junction in a manner that is synchronized with the inputsignal RFin. FIG. 5 shows waveforms that illustrate this relationship inmore detail. Referring to FIG. 5, a periodic input signal RFin yields anoutput current component GmVgs. In addition, the application of thephase-adjusted feedback signal to the MOSFET body results in a dynamicsource-body voltage Vsb that has an amplitude equal to the sum of thephase adjusted feedback signal and the unbiased source-body barrierpotential. Further, the dynamic source-body voltage Vsb is approximately180 degrees out of phase with respect to the input signal RFin.Consequently, when comparing the dynamic source-body voltage waveform tothe waveform of the main drain current component GmVgs, it is seen thatthe value of Vsb has the effect of increasing carrier availability as afunction of the input signal during the period in which the main draincurrent component GmVgs increases from its minimum value to its maximumvalue, and has the effect of decreasing carrier availability as afunction of the input signal during the period when the main draincurrent component GmVgs decreases from its maximum value to its minimumvalue. As a result, application of the phase-adjusted feedback signal tothe MOSFET body amplifies the increase of carrier availability duringthe transition of the drain current from minimum to maximum, andamplifies the decrease of carrier availability during the transition ofthe drain current -from maximum to minimum. This effectively enhancesthe over-all transconductance of the MOSFET. Thus, in contrast to themodel of FIG. 3, the body effect in the MOSFET of FIG. 4 has aconstructive rather than a destructive effect on drain current.Accordingly, the drain current Id of the MOSFET amplifier of FIG. 4 maybe represented as:Id=GmVgs+GmbVbs   (2)

Standard relationships may now be utilized to characterize the operationof the MOSFET in the circuit of FIG. 4. The source voltage of the MOSFETis related to the gate voltage of the MOSFET such that:Vs=χVg, where 0<χ<1 (typically 0.4)Consequently, the gate voltage may be expressed as a function of thegate to source voltage:Vgs=Vg−Vs=Vg−χVg=(1−χ)Vg, or, Vg=1/(1−χ)Vgs   (3)

Further, because the phase-adjusted feedback signal applied to the bodyis derived from the input signal applied to the gate, the body voltageand the gate voltage of the MOSFET are related such that:Vb=βVgwhere β is a feedback factor provided by the phase-adjusting feedbackcircuit. Consequently, the source-body voltage may be expressed as afunction of the gate voltage:Vbs=Vb−Vs=βVg−χVg or, Vbs=(β−ω)Vg   (4)

Substituting equation (3) into equation (4), the relationship betweenVbs and Vgs may be expressed as:Vbs=[(β−χ)/(1−χ)]Vgs   (5)

When the expressions for Gmb and Vbs in equations (1) and (5) aresubstituted into equation (2), an expression for the drain current Id asa function of the gate-source voltage Vgs is obtained: $\begin{matrix}\begin{matrix}{{Id} = {{GmVgs} + {GmbVbs}}} \\{= {{GmVgs} + {{\left( {\alpha\quad{Gm}} \right)\left\lbrack {\left( {\beta - \chi} \right)/\left( {1 - \chi} \right)} \right\rbrack}\quad{Vgs}}}} \\{= {\left\{ {1 + {\alpha\quad\left\lbrack {\left( {\beta - \chi} \right)/\left( {1 - \chi} \right)} \right\rbrack}} \right\}\quad{GmVgs}}}\end{matrix} & (6)\end{matrix}$

Equation (6) shows that the drain current Id produced by the circuit ofFIG. 4 is greater than the drain current that would be produced if thebody was simply connected directly to the source. In other words, thephase-adjusted feedback signal applied to the MOSFET body in the circuitof FIG. 4 enhances the effective transconductance of the MOSFET. Theamount of transconductance enhancement may be varied by varying thefeedback factor β of the phase-adjusting feedback circuit. The feedbackfactor may be set to any value so long as the peak of theforward-biasing phase of the phase-adjusted feedback signal does notexceed the unbiased source-body barrier potential.

While the principal of transconductance enhancement explained above hasbeen illustrated using a feedback signal as illustrated in FIG. 4, itwill be appreciated that this effect may be achieved by applying anysignal to the MOSFET body that has approximately the same waveform asthe input signal and that is shifted by approximately 180 degrees withrespect to the input signal. For example, such a signal may be deriveddirectly from the input signal provided to the MOSFET gate.Alternatively, such a signal may be applied to the MOSFET body by anoscillator circuit that is synchronized to the input signal.

Specific embodiments of the feedback circuit of FIG. 4 are nowdiscussed. FIG. 6 shows an embodiment of the circuit of FIG. 4 in acascode amplifier circuit. As shown in FIG. 6, the cascode amplifierincludes a first n-type MOSFET transistor 60 and a second n-type MOSFETtransistor 62 that are connected in series between a voltage source Vddand ground. The first transistor 60 receives an input signal RFin at itsgate along with a DC bias voltage supplied by a bias circuit 64. Theinput signal RFin is amplified in the first transistor 60 to produced anamplified signal at the drain of the first transistor. The amplifiedsignal is received at the source of the second transistor 62 and isconducted through the second transistor 62 to an output node where anoutput signal RFout is presented.

Unlike the conventional cascode circuit of FIG. 1, the circuit of FIG. 6includes a phase-adjusting feedback circuit that applies aphase-adjusted feedback signal to the body of the amplifying firsttransistor 60. In the implementation shown in FIG. 6, the feedbackcircuit obtains a feedback signal from the gate of the second transistor62, which is modulated by the signal passing between the source anddrain of the second transistor. The feedback circuit is comprised of aresistor 66, capacitor 67 and inductor 68 coupled between Vdd andground, with the gate of the second transistor 62 coupled to a nodebetween the resistor 66 and the capacitor 67, and the body of the firsttransistor coupled to a node between the capacitor 67 and the inductor68. The inductor 68 may be implemented as a down bond inductance that iseffected by connecting the substrate to an available pin on the casingof the circuit package. The values of the resistor 66, capacitor 67 andinductor 68 are chosen such that the signal present at the node betweenthe capacitor 67 and the inductor 68 is phase shifted by approximately180 degrees with respect to the input signal RFin supplied to the firsttransistor 60. The implementation of FIG. 6 is desirable because thesignal obtained from the gate of the second transistor 62 has arelatively small amplitude that is easily attenuated to provide afeedback factor β that is within the acceptable range for application tothe body of the first MOSFET 60. It should be appreciated however thatin alternative embodiments the feedback signal may be derived in othermanners, such as from the signal Rfout at the output node of the secondtransistor 62, and that the components and values of the feedbacknetwork will be chosen accordingly.

A second embodiment of an amplifying circuit in accordance with theinvention is illustrated in FIG. 7. This circuit implements a pair ofcascode circuits of the type shown in FIG. 6 in a differential topology,such that each of the complementary input signals is amplified by arespective cascode amplifier 70, 72 that includes a feedback network forapplying a respective phase-adjusted feedback signal to the body of theamplifying transistor.

A third embodiment of an amplifying circuit in accordance with theinvention is illustrated in FIG. 8. This circuit is similar to thecircuit of FIG. 7 in that it implements a pair of cascode circuits ofthe type shown in FIG. 6 in a differential topology, such that each ofthe complementary input signals is amplified by a respective cascodeamplifier. However this circuit differs from the circuit of FIG. 7 inthat the feedback signals applied to the bodies of the respectiveamplifying MOSFETs 80 a, 80 b of the cascode amplifiers are derived fromthe gate signals of the respective second MOSFETs 82 a, 82 b of theopposite cascode amplifier. In particular, the signal applied to thebody of the amplifying MOSFET 80 a is derived from the gate signal ofthe second MOSFET 82 b of the other cascode amplifier, and the signalapplied to the body of the amplifying MOSFET 82 a is derived from thegate signal of the second MOSFET 80 b of the other cascode amplifier.This implementation may be desirable in some instances because the inputand output signals of the respective cascode amplifiers are phaseshifted with respect to one another by approximately 180 degrees, and soa signal derived from one amplifier may require significantly less phaseadjustment before being applied to the MOSFET body of the otheramplifier, allowing the sizes and values of the feedback circuitcomponents to be reduced.

While the embodiments described herein involve n-type cascodeamplifiers, the methods for transconductance enhancement describedherein may be applied to any MOSFET amplifying circuit. Embodiments ofthe present invention are well-suited for low-noise amplifiers used foramplifying signals in the GHz range, including wireless technologiessuch as wireless LAN transceivers, cellular telephony devices, and otherwireless or handheld devices.

The circuits, devices, features and processes described herein are notexclusive of other circuits, devices, features and processes, andvariations and additions may be implemented in accordance with theparticular objectives to be achieved. For example, circuits as describedherein may be integrated with other circuits not described herein toprovide further combinations of features, to operate concurrently withinthe same devices, or to serve other purposes. Circuits as described mayalso be operable in states not illustrated herein while also beingoperable at different times in the illustrated states. Thus, while theembodiments illustrated in the figures and described above may bepresently preferred for various reasons as described herein, it shouldbe understood that these embodiments are offered by way of example only.The invention is not limited to a particular embodiment, but extends tovarious modifications, combinations, and permutations that fall withinthe scope of the claims and their equivalents.

1. An amplifier circuit comprising: a MOSFET receiving a periodic inputsignal at its gate and amplifying the input signal to produce an outputsignal; and a phase-adjusting feedback circuit receiving a signalcorresponding to the output signal and applying a phase-adjusted signalto the body of the MOSFET, the phase-adjusting feedback circuit causinga phase shift of the received signal such that the phase-adjusted signalapplied to the body of the MOSFET is shifted by approximately 180degrees with respect to the periodic input signal of the MOSFET.
 2. Theamplifier circuit claimed in claim 1, wherein the phase adjustingfeedback circuit comprises: an inductance coupled between the body ofthe MOSFET and ground; and a capacitance coupled between the body of theMOSFET and a node providing the signal corresponding to the outputsignal.
 3. The amplifier circuit claimed in claim 2, wherein theamplifier further comprises a second MOSFET having its source coupled tothe drain of the MOSFET that receives the periodic input signal at itsgate, and wherein the capacitance is coupled to the gate of the secondMOSFET
 4. A cascode amplifier circuit comprising: a first MOSFET; asecond MOSFET having its source connected to the drain of the firstMOSFET; and a feedback circuit coupled between the gate of the secondMOSFET and the body of the first MOSFET.
 5. The cascode amplifiercircuit claimed in claim 4, wherein the feedback circuit produces aphase shift in an input signal received from the gate of the secondMOSFET such that an output signal supplied to the body of the firstMOSFET is shifted by approximately 180 degrees with respect to an inputsignal received at the gate of the first MOSFET.
 6. The cascodeamplifier circuit claimed in claim 4, wherein the feedback circuitcomprises a capacitance coupled between the gate of the second MOSFETand the body of the first MOSFET, and an inductance coupled between thebody of the first MOSFET and a ground potential.
 7. The cascodeamplifier circuit claimed in claim 6, further comprising: a voltagesource supplying a voltage to the drain of the second MOSFET; and aresistor coupled between the gate of the second MOSFET and the voltagesource.
 8. A differential cascode amplifier circuit comprising: a firstMOSFET; a second MOSFET having its source connected to the drain of thefirst MOSFET; a third MOSFET; a fourth MOSFET having its sourceconnected to the drain of the third MOSFET; a first feedback circuitcoupled between the gate of the second MOSFET and the body of the firstMOSFET; and a second feedback circuit coupled between the gate of thefourth MOSFET and the body of the third MOSFET.
 9. The differentialcascode amplifier circuit claimed in claim 8, wherein the first feedbackcircuit produces a phase shift in an input signal received from the gateof the second MOSFET such that an output signal supplied to the body ofthe first MOSFET is shifted by approximately 180 degrees with respect toan input signal received at the gate of the first MOSFET, and whereinthe second feedback circuit produces a phase shift in an input signalreceived from the gate of the fourth MOSFET such that an output signalsupplied to the body of the third MOSFET is shifted by approximately 180degrees with respect to an input signal received at the gate of thethird MOSFET.
 10. The differential cascode amplifier circuit claimed inclaim 8, wherein the first feedback circuit comprises a capacitancecoupled between the gate of the second MOSFET and the body of the firstMOSFET, and an inductance coupled between the body of the first MOSFETand a ground potential, and wherein the second feedback circuitcomprises a capacitance coupled between the gate of the fourth MOSFETand the body of the third MOSFET, and an inductance coupled between thebody of the third MOSFET and a ground potential.
 11. The differentialcascode amplifier circuit claimed in claim 10, further comprising: avoltage source supplying a voltage to the drains of the second andfourth MOSFETs; a first resistor coupled between the gate of the secondMOSFET and the voltage source; and a second resistor coupled between thegate of the fourth MOSFET and the voltage source.
 12. A differentialcascode amplifier circuit comprising: a first MOSFET; a second MOSFEThaving its source connected to the drain of the first MOSFET; a thirdMOSFET; a fourth MOSFET having its source connected to the drain of thethird MOSFET; a first feedback circuit coupled between the gate of thesecond MOSFET and the body of the third MOSFET; and a second feedbackcircuit coupled between the gate of the fourth MOSFET and the body ofthe first MOSFET.
 13. The differential cascode amplifier circuit claimedin claim 12, wherein the first feedback circuit produces a phase shiftin an input signal received from the gate of the second MOSFET such thatan output signal supplied to the body of the third MOSFET is shifted byapproximately 180 degrees with respect to an input signal received atthe gate of the third MOSFET, and wherein the second feedback circuitproduces a phase shift in an input signal received from the gate of thefourth MOSFET such that an output signal supplied to the body of thefirst MOSFET is shifted by approximately 180 degrees with respect to aninput signal received at the gate of the first MOSFET.
 14. Thedifferential cascode amplifier circuit claimed in claim 12, wherein thefirst feedback circuit comprises a capacitance coupled between the gateof the second MOSFET and the body of the third MOSFET, and an inductancecoupled between the body of the third MOSFET and a ground potential, andwherein the second feedback circuit comprises a capacitance coupledbetween the gate of the fourth MOSFET and the body of the first MOSFET,and an inductance coupled between the body of the first MOSFET and aground potential.
 15. The differential cascode amplifier circuit claimedin claim 14, further comprising: a voltage source coupled to the drainsof the second and fourth MOSFETs; a first resistor coupled between thegate of the second MOSFET and the voltage source; and a second resistorcoupled between the gate of the fourth MOSFET and the voltage source.16. A method of producing an amplified signal comprising: providing afirst signal as an input signal at the gate of a MOSFET; applying asecond signal to the body of the MOSFET, the second signal havingapproximately the same waveform as the first signal and being shifted byapproximately 180 degrees with respect to the first signal; andproviding an output signal at a drain of the MOSFET in response to thefirst signal and the second signal.
 17. The method circuit claimed inclaim 16, wherein a peak voltage amplitude of the second signal does notexceed an unbiased source-body barrier potential of the MOSFET.
 18. Themethod circuit claimed in claim 16, wherein the second signal isgenerated by a feedback circuit that receives as input an output signalof the MOSFET.
 19. The method circuit claimed in claim 16, wherein thesecond signal is generated by a feedback circuit that receives as inputan output signal of a second MOSFET coupled in a cascode fashion to theMOSFET.
 20. The method circuit claimed in claim 16, wherein the secondsignal is generated by a feedback circuit that receives as input a gatesignal of a second MOSFET coupled in a cascode fashion to the MOSFET.21. The method circuit claimed in claim 16, wherein the second signal isgenerated by an oscillator circuit synchronized with the first signal.